A high-performance sports car normally has a rear-wheel drive, and a self-locking differential to maximize torque transmission by the rear wheels to the road surface in any driving condition.
A rear-wheel drive with a self-locking differential has advantages when driving in racing mode in good (dry-road) or fairly good (wet-road) grip conditions, but, in conjunction with high torque and wide tyres, makes for extremely difficult, potentially hazardous driving in poor grip conditions (flooded or icy roads). To improve handling of sports cars in poor grip conditions, a permanent or optionally connectable four-wheel drive has been proposed.
A permanent four-wheel drive greatly improves vehicle performance in poor grip conditions, but has the drawback of permanently increasing torque loss of the drive system and so resulting in good-grip vehicle performance which is not always popular with drivers.
An optionally connectable four-wheel drive, on the other hand, enables the driver to switch between a rear-wheel drive in good grip conditions, and a four-wheel drive in poor grip conditions.
Patent Application WO2005080117A2describes an optionally connectable four-wheel drive vehicle comprising an engine with a drive shaft; two main drive wheels connected permanently to the drive shaft via a transmission with a first clutch; and two secondary drive wheels selectively connectable to the drive shaft by an optionally connectable drive system having a second clutch which is connected on one side with a fixed velocity ratio to the drive shaft upstream from the transmission, and is connected on the other side with a fixed velocity ratio to the secondary drive wheels.
Patent Application WO2006100585A1describes a refinement of the optionally connectable four-wheel drive vehicle in Patent Application WO2005080117A2. More specifically, the percentage value of the torque to be transmitted to the secondary drive wheels by the second clutch is determined instant by instant as a function of dynamic vehicle parameters detected by respective sensors.
In the solutions proposed in Patent Applications WO2005080117A2 and WO2006100585A1, for the second clutch to transfer torque to the front differential (i.e. to the secondary front drive wheels), the front differential, i.e. the output of the second clutch, must be slower than or at the same speed as the input of the second clutch. In fact, if the second clutch were to be engaged when its output is faster than its input, the second clutch would transfer torque from the front differential to the transmission, i.e. to the rear drive wheels, thus braking the front wheels and overtorquing the rear drive wheels.
The second clutch has unavoidable design limitations, by being unable to ensure an adequate working life if it has to operate with too great a difference in speed between its input and output. As a result, in the solutions proposed in Patent Applications WO2005080117A2and WO2006100585A1, the velocity ratios of the system must be such that:                when the third gear is engaged, the secondary shaft of the transmission (governing rotation of the rear wheels), the output of the second clutch (governing rotation of the front wheels), and the input of the second clutch have the same angular speed, so the second clutch can be fully engaged (i.e. with no slip) to transfer torque to the front wheels;        when the first or second gear is engaged, the secondary shaft of the transmission and the output of the second clutch are slower than the input of the second clutch, so the second clutch can be engaged partly (i.e. with a certain amount of slip to match speed) to transfer torque to the front wheels;        when a higher-than-third gear is engaged, the secondary shaft of the transmission and the output of the second clutch are faster than the input of the second clutch, which therefore cannot be engaged to transfer torque to the front wheels.        
As a result, in the solutions proposed in Patent Applications WO2005080117A2and WO2006100585A1, the four-wheel drive can only be engaged in low gears (typically, first, second, and third), and not at all in the other gears.